Field of the Invention
The present invention relates to an image forming apparatus that forms an image on a recording material using an electrophotographic scheme.
Description of the Related Art
Conventionally available electrophotographic image forming apparatuses include electrophotographic copiers, electrophotographic printers (LED printers, laser beam printers, and the like), and electrophotographic facsimile machines. In these image forming apparatuses, a surface of an electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum or a drum) is uniformly charged using a primary charger and the charged photosensitive drum surface is exposed using an exposure apparatus, thereby forming an electrostatic latent image. The electrostatic latent image is developed using a developing apparatus to form a developer image (hereinafter referred to as a toner image). The toner image is transferred to a recording material, such as a sheet, using a transfer apparatus. Subsequently, the toner image is fixed on the recording material as a fixed image using a fixing apparatus, and the fixed image is then output. Thereafter, the photosensitive drum is cleaned by a cleaning apparatus removing the toner having remained on the surface of the photosensitive drum after the transfer of the toner image, and thus the image forming apparatus is prepared for the next image forming operation.
In recent years, more and more image forming apparatuses have incorporated a charging apparatus employing contact charging, and such contact charging apparatuses have become prevailing. Most of the contact charging apparatuses use a conductive roller as a contact charging member and employ roller charging in which a voltage is applied by bringing the conductive roller into contact with the photosensitive drum. Two charging schemes are available; one of the schemes is a direct-current scheme in which the photosensitive drum surface is charged by applying only a direct-current voltage to the contact charging member, and the other scheme is an alternating-current superimposition scheme in which the photosensitive drum surface is charged by superimposing an alternating-current voltage on a direct-current voltage and applying the resultant current to the contact charging member. The alternating-current superimposition scheme has an advantage in uniformly charging the photosensitive drum surface. However, in the alternating-current superimposition scheme, discharge occurs frequently in accordance with the frequency of an alternating-current voltage, causing damage to the photosensitive drum surface and an increase in the amount by which the photosensitive drum surface is scraped off. This shortens the life of the photosensitive drum. In contrast, in the direct-current scheme, discharge in fine gaps occurs less frequently than in the alternating-current superimposition scheme, causing less damage to the photosensitive drum, which results in a longer life of the photosensitive drum.
Meanwhile, a conventionally known transfer step by the image forming apparatus involves bringing a transfer member such as a transfer roller into abutting contact with the photosensitive drum to form a transfer nip. Specifically, the toner image on the photosensitive drum is transferred to the recording material by applying a transfer bias to the transfer member while passing a recording material or an intermediate transfer member through the transfer nip. Resistance of the transfer member used in the transfer step is known to fluctuate according to the temperature and humidity of an atmospheric environment or over a long period of use. To allow high transferability to be constantly achieved in spite of the fluctuations in resistance, Japanese Patent Application Laid-open No. 2010-26442 proposes constant-voltage control based on active transfer voltage control (ATVC). Specifically, before image formation is started, a voltage with a predetermined value is applied to the transfer member, and a current output current value is sensed. Based on the applied voltage and the sensed current value, the value of the resistance between the transfer member and the photosensitive member is determined. Then, in accordance with the result of the determination, the value of the transfer voltage applied to the transfer member during subsequent image formation is adjusted.
In this ATVC, a slight variation in photosensitive drum potential affects the sensed voltage, and thus, the photosensitive drum surface needs to maintain a uniform potential during detection. Normally, this is performed where the potential of the photosensitive drum is a dark potential (Vd). The Vd potential portion is used because if a photosensitive layer in a photosensitive member surface is strongly charged to a polarity opposite to a regular polarity (in the present embodiment, a negative polarity) using the transfer member, a memory with an opposite polarity (referred to as a plus memory) is formed in the photosensitive layer surface, which may thus be damaged. The likelihood of the photosensitive layer surface being damaged increases with decreasing potential on the photosensitive member, i.e. The photosensitive layer surface is likely to be damaged at an exposure potential VL. However, residual charge in the last formed image pattern may vary the drum potential of the charged photosensitive drum, reducing the accuracy of the ATVC. Thus, a method may have been used in which the surface of the photosensitive drum is uniformly irradiated with light from a pre-charge exposure apparatus having a light source such as LEDs to make the potential of the photosensitive drum uniform before charging. This prevents a possible variation in the drum potential of the charged photosensitive drum.
However, there has been an increasing demand for an extended life of the photosensitive drum in order to reduce running costs of the image forming apparatus. The direct-current scheme advantageously reduces the amount by which the photosensitive drum surface is scraped off. However, the photosensitive drum surface is also degraded by discharge, and is scraped off as a result of passage of sheets or abutting contact with a cleaning member. The film thickness of the photosensitive drum thus decreases steadily. Furthermore, the pre-charge exposure makes the drum potential uniform to improve the accuracy of the ATVC, but reduces a surface potential of the uncharged photosensitive drum. This increases the amount of discharge and thus the amount by which the photosensitive drum surface is scraped off.
To avoid this problem, the initial film thickness of the photosensitive drum may be increased and the preliminary exposing or pre-exposure may be carried out immediately before execution of the ATVC, in order to minimize an irradiation time for the pre-charge exposure. However, abnormal discharge may conventionally occur when an area subjected to the pre-charge exposure is charged, resulting in a non-uniform potential of the charged drum to reduce the accuracy of the ATVC. This feature is conspicuous when the initial film thickness of the drum is increased in order to extend the life of the photosensitive drum.